Measurement of directional information in a sound field is frequently of great interest. “Directional information” is meant to refer to characteristics of the angular distribution of sound passing through a point. Such information is not readily available through observation of the pressure or intensity alone. The sound pressure is a non-directional measure, whereas intensity is a vector indicating the net direction of energy flow, not necessarily the direction of arrival of component sound waves. Application areas in which knowledge of directional properties of sound fields could be useful include room acoustic analysis and characterization, psycho-acoustic assessment of halls or localization of sources and reflections to name just a few.
A straightforward approach at obtaining directional information is to employ a detector that is responsive to sounds arriving from one direction only. A directional detector could mean a single directional transducer, a shotgun microphone, a parabolic microphone, or a microphone array for instance. Performance issues (such as angular resolution, bandwidth, fidelity) and practical issues (such as ease of steering in different directions, size, cost) together dictate what type of detector is desirable.
Beamforming microphone arrays have many favorable properties for directional pickup of sound. They can be designed to yield high directionality, a broad frequency range of operation, and can be steered electronically in many directions simultaneously, without the need for movement of the array.
With modern microphones and digital acquisition hardware, highly sophisticated arrays can be realized quite inexpensively. However, in the case of this invention they can adapt instantaneously to any sound originating from any direction in the soundfield.
Choice of suitable array geometry is an issue. If the goal is to design a directional detector for analyzing sound fields (as in the present work), then in many instances one desirable attribute is spherical symmetry. A spherical array can enable steering an identical beam in any three-dimensional direction. Other three dimensional arrays such as hemisphere or ellipsoids are also possible. Linear or planar arrays do not provide the same functionality.
Beamformer design has developed extensively in the past 50 years or so. Delay-and-sum designs are simple and robust, but only provide maximum directional gain over a narrow frequency range. Superdirective approaches can achieve higher directional gain over a wider frequency range, but at the expense of simplicity and robustness. The signal-to-noise ratio becomes a problem at low frequencies, where the phase change of the sound waves is small over the spatial extent of the array. At higher frequencies, the wavelengths become shorter than the inter-microphone spacing, causing problems with spatial aliasing. General tradeoffs in achieving higher directionality over a broader frequency range include: tighter required microphone tolerances, less noise immunity, and possibly more difficult construction issues.
The utility of microphone arrays is based on the principle that all acoustic events can be represented by four basic elements. These are ‘X’ which is front/back information (depth), ‘Y’ which is left/right information (width), ‘Z’ which is up/down information (height) and ‘W’ the central point from which the other three elements are referenced.
Advanced arrays capture three dimensional sound at the same ‘central point’ so all time/or phase-related anomalies created by spaced microphones are eliminated.
U.S. Pat. No. 5,778,083 to Godfrey discloses a microphone array used for surround sound recording. It utilizes a frame for mounting linear pick up microphones such that each of the microphones has its diaphragm facing outwards from the frame, and the diaphragms form a generally elliptical pattern. It is stated that the shape must be non-circular.
U.S. Pat. No. 6,041,127 to Elko discloses a microphone array consisting of 6 small pressure-sensitive omni-directional microphones mounted on the surface of a small rigid nylon sphere. DSP is used to derive sound output.
U.S. Pat. No. 4,675,906 to Sessler and West discloses a microphone array using a cylinder with open ends in which four bi-directional microphones are mounted at 90 degree intervals on the wall of the cylinder, providing a toroidal pick-up pattern. The partially open nature of the cylinder allows the reception of sound waves transversing the cylinder to be received at different intensities.
U.S. Pat. No. 6,851,512 to Fox et al. discloses a microphone array using a modular structure capable of varying configurations, all having closed surfaces.